The present invention relates to the treatment of the surfaces of blood-contacting materials and devices to reduce the thrombogenicity of the materials.
Many modern surgical and other medical procedures, sampling techniques and test procedures involve the use of blood-contacting devices, such as catheters, drains and extracorporeal circuitry. These devices are used only once and then discarded for hygiene reasons and must therefore be constructed from the most economical materials available, usually polymeric plastics or glass. However glass and most synthetic and natural polymers tend to induce platelet aggregation and initiate the fibrinolytic clotting cascade leading to blockage of tubing and clogging of other apparatus such as filtration and dialysis membranes, interference with test procedures and, in certain cases, may have disastrous consequences for patients.
With existing technology, extra-corporeal circulation for haemodialysis, long term gas exchange at states of severe respiratory failure and cardiac support, e.g. after cardiac surgery, all require systemic heparinisation. Owing to the risk of excessive bleeding after systemic anticoagulant treatment many patients are disqualified for possible therapeutic measures. Likewise, commercial catheter sensors, e.g. for continuous determination of arterial oxygen, carbon dioxide tension, and of pH in the critically ill patient require systemic heparinisation to prevent microclotting on the sensor membranes and failure of the device.
It has been suggested that heparinisation of apparatus, particularly involving end point attachment of heparin fragments, will result in anti-thrombogenic coating of surfaces and thereby overcome the need for systemic heparinisation. Thus a method was developed in which heparin was coupled by end point attachment [Hoffman, J. et al., Carboyhdr, Res., 117:328(1983), Larm, O. et al., Biomat. Med. Dev. Art. Org., 11:161(1983)]. The resulting surfaces adsorbed antithrombin and large amounts of thrombin which were rapidly inhibited in the presence of antithrombin [Pasche, B, et al., Thromb. Res., 44 739(1986)]. It is interesting to note that end-point attached heparin and the endothelium behave both quantitatively and qualitatively alike with respect to the inhibition of thrombin in the presence of plasma [Arnander C., et al., J. Biomed. Mat. Res., 20:235(1986)] and that a polyethylene surface furnished with end-point attached heparin showed considerable capacity to inhibit Factor Xa [Kodama, K. et al. Thromb. Haemostas. (1987)].
Rigid polyethylene tubing sections with end-point attached heparin have been kept in the thoracic aorta of pigs for up to four months [Arnander C., et al., Biomat. Res., (1987)]. When applied to vascular grafts of expanded polytetrafluoroethylene (PTFE) and to polyurethanes and implanted in the arteries of the sheep [Esquivel, C. O. et al., Surgery, 95:102(1984)], the end-point attached heparin surface substantially reduced the platelet and fibrin deposition.
The extra-corporeal circulation of blood through surface-heparinised devices offered the possibility to discriminate between the role of platelets and the plasma coagulation system as the main determining factor for achieving thromboresistance. In these experiments, it was demonstrated that coatings with other sulfated polymers were as platelet compatible as the heparin coatings, but still thrombogenic. Using the radioimmunoassay for fibrinopeptide A [Nossel, H. L. et al., J. Clin. Invest., 54:43(1974)] it was shown that only coatings on which the heparin molecules could interact with plasma constituents were able to prevent conversion of fibrinogen to fibrin on contact with blood [Larsson R. and Lindahl U. Artif. Org., Vol 3. Suppl. Proc. of the second meeting of the Intern. Soc. Artif. Org., (1979); Larsson R., et al., Thromb. Res., 19:43(1980); Larm, O. et al., Biomat. Med. Dev. Art. org., 11:161(1983)].
Thus the presence of intact functional groups on the immobilised heparin appeared mandatory for achieving thromboresistance and heparin coatings on blood-contacting medical devices could eliminate hazardous systemic anticoagulant treatment.
Experimental haemodialysis has been performed on dogs without systemic heparinisation and with cellulose acetate hollow fibers filters with end-point attached heparin surfaces. The efficiency of the coating on the total extra-corporeal system was demonstrated by the fact that the levels of fibrinopeptide A in the dialysed animals were not higher than in anaesthetised control animals with no surgery [Arnander C., et al., Proc. Eur. Soc. Art. Org., 9:312(1982), Lins, L. E et al., Proc. EDTA-ERA, 21:270(1984). When end-point attached heparin surfaces were used in the extra-corporeal circuit, veno-venous by-pass for carbon dioxide elimination was easily performed for 24 h on dogs in a steady-state condition. After a small release of heparin, the coagulation system seemed unaffected as determined by fibrinopeptide A levels in the circulating blood [Larm, O. et al., An approach to antithrombosis by surface modification. Progress in artificial organs, ISAIO Press, Cleveland 1986, p313. Inacio, J. et al., Extracorporeal elimination of carbon dioxide using a surface heparinised vein-to-vein bypass system. EUROXY Workshop on design and techniques of extracorporeal gas exchange. Paris, Jun. 20, 1985. Bindsley L., et al., Trans. Am. Soc. Art. Int. Org. 32:530(1986).
Although heparinisation can reduce or prevent fibrinolytic clotting, this is at the expense of interference with the blood biochemistry, for instance the complexation of antithrombin and other subtle alterations. The heparinised surfaces are exerting many of the effects of heparin when administered as an anti-coagulant drug and the adverse side effects of heparin must, therefore, be taken into account when this technique is employed to improve haemocompatibility of blood-contacting devices.
Much effort has been devoted in recent years to the development of surface treatments, especially by covalent bonding of haemocompatible organic groups which improve the biocompatibility of blood-contacting surfaces and to the production of more biocompatible materials for use in blood-contacting devices such as surgical implants, prostheses and artificial hearts (see, for instance EP-A-0032 622 and EP-A-O 157 496). However, being intended to provide long term biocompatibility, these tend to be relatively expensive and are therefore less suitable for use in low-cost, single-use disposable devices.
The present invention provides a simple and cost effective means for reducing the thrombogenicity of blood-contacting surfaces using readily available materials.
The coatings of the present invention do not interfere with blood biochemistry in any manner and are therefore regarded as non-thrombogenic rather than as anti-thrombogenic. In addition to avoiding the side-effects of heparinisation, the present treatment offers the further advantages in many applications in that the coated surfaces have improved wetability and improved lubricity. This assists in, for instance, avoiding the formation of gas bubbles in tubing and facilitating insertion of catheters via surgical incisions.
According to the invention there is provided a process for coating a blood-contacting surface comprising applying a solution of a fatty acid diester of phosphatidyl choline in an organic solvent to the surface and removing the solvent.
FIG. 1 shows a comparison between a piece of medical tubing treated using the process of the invention and a similar tube of untreated medical tubing.
The surface to be treated may optionally be prepared for coating by washing to remove surface contaminants and, to improve the adhesion of the coating, by silylation or otherwise increasing the hydrophobicity of the surface.
The thickness of the coating will be selected according to the intended use of the blood-contacting device. Thus surfaces subjected to shear forces, such as tubing carrying a flow of blood, will generally receive thicker coatings than those for static applications such as sample phials or holders. The thickness of the coating will also be selected according to the intended duration of use of the coated device since thicker coatings will last longer. Typical thicknesses envisaged for coatings according to the invention are in the order of 1 to 1000 nanometres, preferably 10 to 500 nanometres and most preferably about 100 nanometres.
The blood-contacting surface may be a surface of a finished blood-contacting device or it may be the surface of a material to be used in forming a blood-contacting device. In the latter case subsequent forming steps are selected to avoid disrupting the coating formed by the process of the invention in blood-contacting portions of the device and to avoid chemical damage, for instance due to high temperatures, to the phosphatidyl choline ester. The surface being treated will hereinafter be referred to as the xe2x80x9csubstratexe2x80x9d.
Substrates which may be coated according to the invention include glasses (eg soda glass and silica glass), metals such as silver, natural polymers, such as cellulose and semisynthetic cellulose derivatives, and artificial polymers such as polyurethanes (eg Pellethane), vinyl polymers such as polyvinyl chloride (PVC), polyesters such as polyethylene terephthalate (eg Dacron), polyalkenes such as polyethylene and polypropylene, polycarbonates and fluoropolymers such as polytetrafluoroethylene (PTFE) and other poly(fluoroalkenes) including fluorinated ethylene polymer (FEP) which is a copolymer of tetrafluoroethylene and hexafluoropropylene.
The fatty acid diesters of phosphatidyl choline which may be used in the process of the invention include esters of saturated and unsaturated fatty acids and may be pure single compounds such as dipalmitoyl phosphatidyl choline (DPPC) and dimyristoyl phosphatidyl choline (DMPC), mixtures of such compounds and purified natural products such as the fatty acid diesters of phosphatidyl choline from egg yolk or soya bean lecithin. Mixed diesters of phosphatidyl choline may be used. Preferably the fatty acid side chains will be straight as opposed to branched and will have from 12 to 20 carbon atoms. Purified natural products may contain a small proportion of components other than fatty acid diesters of phosphatidyl choline but these should not be present in sufficient amount to impair the thrombogenicity-reducing effect of the coating. In particular, phosphatidyl serine, and other anionic phospholipids which would cause clotting, should be avoided.
The solvent may be any conventional organic solvent which will dissolve the saturated fatty acid diester of phosphatidyl choline. Preferably the solvent will be selected for lack of toxicity and environmental hazards, for ease of removal, for compatibility with the material to be treated and for pharmacological acceptability. The solvent may be one which swells a polymeric substrate and this will aid the treatment process by permitting the phosphatidyl choline diester to penetrate the surface of the substrate. Alternatively the solvent may be selected to avoid swelling of a polymer substrate, particularly where there are fine dimensional tolerances to be maintained or other useful properties of the polymer would be thereby impaired.
Preferred solvents include the lower alkanols such as methanol, ethanol and iso- or n-propanol, halogenated alkanes such as chloroform and mixtures thereof. A particularly preferred solvent is a mixture of ethanol and chloroform, such as from 20:1 to 80:1 preferably 40:1 ethanol:chloroform by volume. Another preferred solvent is a mixture of Freon and ethanol such as from 50:50 to 99:1, preferably 90:10 Freon: ethanol by volume.
The concentration of phosphatidyl choline diester in the solvent will be selected to avoid use of unduly large quantities of solvent (and ensuing technical difficulties with removal thereof and economic penalties) whilst enabling efficient coating of the substrate. Preferred concentrations are in the range of from 0.5 to 20 mg/ml, preferably 2, 5 or 10 mg/ml.
The solution of phosphatidyl choline diester may be applied by any conventional coating technique such as immersion in a coating bath, spraying, painting or, for flat substrates, spin-coating, the coating conditions being varied according to the desired thickness of coating and the hydrophobicity of the substrate since the phosphatidylcholine diesters adhere more strongly to more hydrophobic substrates. Preferably coating is achieved by immersion of the substrate in a bath of a phosphatidyl choline fatty acid diester solution in a suitable solvent at an appropriate concentration and temperature for sufficient time to cover the surfaces to be coated.
The solvent may be removed by conventional techniques, preferably by evaporation under reduced or ambient pressure, in a gas stream and/or at elevated temperature. By careful selection of the solvent, concentration of the solution and coating and solvent-removal techniques, the thickness of the phosphatidyl choline diester coating may be controlled within a desired range.
Particularly preferred solvents, concentrations and coating and solvent removal techniques are described below and illustrated by the Examples.
Pre-washing of the surface may be effected using a suitable solvent such as those described above which may be the same as or different to the solvent or solvent system used to apply the coating. Pre-treatment of the surface by silylation is preferably effected using a reactive alkyl silane such as a halo silane, for instance trichlorooctadecyl silane or chlorodimethyl octadecylsilane in a suitable solvent such as hexane or chloroform. Excess reagent may be removed by a further washing step.
It will be appreciated that for some surfaces, such as glass, it will be necessary to pre-treat the surface for example by silylation to obtain a surface to which the fatty acid diester of phosphatidyl choline will adhere.
The present invention further provides a process for reducing the thrombogenicity of a blood-contacting surface comprising forming a coating on the surface of a fatty acid diester of phosphatidyl choline.
The invention also provides a material having a non-thrombogenic coating of a fatty acid diester of phosphatidyl choline on at least a portion of the surface thereof. Preferably the material is, or is a part of, a blood-contacting device. Typical blood contacting devices include tubing such as catheters, for instance central venous catheters, thoracic drain catheters, and angioplasty balloon catheters, tubing used in extracorporeal circuitry such as in heart and/or lung bypasses and entire extracorporeal circuits such as whole blood oxygenators, cannulae, vascular grafts, sutures, membranes such as those used in blood separation, apheresis and donorpheresis units, gas exchange membranes such as used in whole blood oxygenators, polycarbonate membranes and haemodialysis membranes and membranes used in diagnostic and biosensor devices, biosensors and other devices used in diagnosis such as cuvettes used in blood clotting time determinations. A particular example of a blood contacting device which may be treated according to the invention is a PVC thoracic drain catheter.
FIG. 1 shows a comparison between a piece of medical tubing treated by the process of the invention and a similar untreated piece of tubing. In each case the tubing has been exposed to a flow of blood. In the case of the untreated tube the formation of blood clots in the tube leading to blockage of the tube can be seen whereas the tube treated according to the invention is free of clots. Thus the presence of a non-thrombogenic surface inside the tube which was treated according to the invention is demonstrated.